Earth's magnetic field gathers momentum

Physicists in France have linked subtle variations in the length of day with conditions in the Earth's core – where the Earth's magnetic field originates. The finding could improve our poor understanding of how the field is generated and why it changes in response to conditions deep within the Earth's interior.

Molten iron flowing in the outer core generates the Earth's geodynamo, leading to a planetary-scale magnetic field. Beyond this, though, geophysicists know very little for certain about the field, such as its strength in the core or why its orientation fluctuates regularly. Researchers do suspect, however, that field variations are strongly linked with changing conditions within the molten core.

As we cannot access the Earth's core directly, researchers look to clues at the Earth's surface. One intriguing suggestion is that changing conditions at the core could have an impact on angular momentum throughout the whole Earth system. The implication is that variation to the flow patterns in the core could have an impact on the Earth's rotation, which could lead to slight variations in the length of a day.

New wave

Nicolas Gillet and colleagues at the Université Joseph Fourier claim to have the strongest evidence yet that this is indeed happening. By reconstructing flow within the Earth's core using an established model of the geodynamo, the researchers see a type of wave – called an Alfven wave – emerge from within the core. They believe that this wave, not seen before in simulations, is transferring angular momentum through the core towards the overlying mantle.

Closer inspection of the simulations revealed that these Alfven waves are dragged by the magnetic field and they recur just once every six years. The key result is that this periodicity corresponds with a six-year signal in the variation to the length of day, leading the researchers to link the two phenomena. They argue that the Alfven waves play a role in balancing angular momentum throughout the Earth. "When the core rotates faster, the rotation of the mantle must be slower in order to compensate, which in turn increases the length of day," explains Nicolas Gillet.

Having established this link, Gillet's team focused their attention on the Alfven wave as it propagates through the core. Realizing that the wave takes approximately four years to reach the mantle, they were able to calculate the strength of the Earth's magnetic field within the core – approximately 4 mT. This value is the most reliable yet for the magnetic field in the core, claim the researchers.

Good value

Ulrich Christensen, a geophysicist at the Max Planck Institute for Solar System Research is impressed by the unified approach taken by Gillet's team. "I like the value derived from this analysis as it is in line with what I would expect from the recent geodynamo simulations," he says.

Previous estimates of the magnetic field within the core had come directly from numerical simulations, or from interpreting geomagnetic data gathered at the surface. "Our study revisits the estimate from geophysical data, and reconciles it with geodynamo simulations," says Gillet.

And the full significance of this research may not be realized yet. The researchers believe that they can go on to develop a more complete model of the geodynamo and the way angular momentum is transferred through the core. "It is important in order to understand how the geodynamo works and how this is linked with the thermal history of the planet," says Gillet.

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7 comments

Earth's & Sun's Magnetic Fields Are Linked

The Earth probably inherited its magnetic field from the highly magnetic object at the center of the solar system as the Sun was accreting back onto the collapsed core of the supernova that gave birth to the solar system [Science 195 (1977) 208-209; Nature 277 (1979) 615-620].

The seven stable molybdenum isotopes (Mo-92, Mo-94, Mo-95, Mo-96, Mo-97, Mo-98 and Mo-100) in iron meteorites have not been throughly mixed since they were produced by stellar nuclear reactions, as first reported by Qi-Lu [Doctoral Dissertation (1991) University of Tokyo] and subsequently confirmed by several others, e.g. Q.Z. Yin et al. [Nature 415 (2002) 881-883]

Magnetic field, core changes, angular momentum..

It would be nice to see the external magnetic field, and the changes caused to it by the IMF included in this picture. The changes at the core are not all self-caused, and there are both smaller and larger scales to consider.

Alfvén waves? You learn something new every day! I was interested to read on wiki that "at high field or low density, the velocity of the Alfvén wave approaches the speed of light, and the Alfvén wave becomes an ordinary electromagnetic wave".For some reason this makes me think about the Einstein-de Haas effect, which "demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". Interesting stuff.

Magnetic Pole Motion

The Earth's North Magnetic Pole is changing from almost fixed location in the 1700's to movement with increasing velocity and, maybe, even increasing acceleration as well as decreasing field strength and is now out over the Arctic Ocean heading towards Siberia. The most important question would be whether the magnetic poles are headed towards a "Flip" as has happened in its history but, in the short term, does this change affect the 6-year cycle and changes in length of day of the article herein ??

I have question as regards the discovery. Could this be a different sets of waves different from the gravitational waves that have long been predicted by many writers but yet to be discovered or detected in the laboratory? Can we say in affirmation that we have arrived at the detection of gravitational waves?